Layering nitrided oxide on a silicon substrate

ABSTRACT

A process for producing a nitrided oxide layer on a silicon semiconductor substrate includes introducing a multiplicity of wafers into an atmospheric batch furnace, carrying out an oxidation step at a first predetermined temperature, carrying out a nitriding step at a second predetermined temperature, and carrying out a reoxidation step at a third predetermined temperature. The wafers are then cooled and removed from the atmospheric batch furnace.

RELATED APPLICATIONS

This application claims the benefit of the Jul. 11, 2001 priority dateof German application 101 33 537.7, the contents of which are hereinincorporated by reference.

FIELD OF INVENTION

The present invention relates to a process for producing a nitridedoxide layer on a silicon semiconductor substrate.

BACKGROUND

Although it can in principle be applied to any desired oxides, thepresent invention and the problem on which it is based are explainedwith reference to tunnel oxides in integrated DRAM circuits used insilicon technology.

To produce nitrided tunnel oxide, for example for flash memorycomponents, the defined inclusion of nitrogen close to thesilicon/silicon oxide interface is necessary, in order to satisfy thedemands imposed with regard to long oxide functionality (frequenttunneling, very numerous write/erase cycles).

Furthermore, as part of the ongoing miniaturization of these integratedcircuits, it is necessary to maintain a low thermal budget of thisprocess, in order to keep the outdiffusion of the dopants which havealready been introduced by this stage of this processing at a low level.

For a narrow yield distribution at a high level, in particular theuniformity of the oxide thickness and of the inclusion of nitrogen areimportant.

FIG. 3 shows a diagrammatic illustration of a semiconductor substratewith a nitrided tunnel oxide in order to illustrate the problem on whichthe present invention is based.

In FIG. 3, 1 denotes a silicon semiconductor substrate, 10 denotes afirst active region, 20 denotes a second active region, 15 denotes achannel region between them, 30 denotes a nitrided tunnel oxide lyingabove the channel region 15 and 40 denotes a standard gate stack, whichincludes, for example, a floating gate and a control gate.

The region A is illustrated separately on an enlarged scale. In thisenlarged illustration, G denotes the interface between the siliconsemiconductor substrate 1 and the nitrided tunnel oxide 30. The definedinclusion of nitrogen in the crystal lattice of the silicon dioxide isclearly visible.

FIG. 4 shows an example of a temperature curve for a standard RTPprocess used to produce a nitrided tunnel oxide.

In the example shown, the nitriding takes place by means of NH₃, withthe result that it is possible to produce tunnel oxides with aparticularly high cycle stability of typically 10⁶ write/erase cycles.The RTP process requires lamp heating in the individual wafer process,in which the individual silicon wafers are brought to the appropriateprocess temperature and are successively oxidized, nitrided andreoxidized using fluctuating temperatures and gas flows. The reoxidationprocess is used in particular to remove hydrogen.

The productivity of such a process is very low, since the processing ofa single wafer lasts a few minutes. To achieve viable process results atleast within this time, very high process temperatures are required,which leads to a high temperature-time load on the process wafers. Sincethe processed wafer and the process chamber are not in thermodynamicequilibrium throughout the entire process, the temperature distributionover the wafer can only be monitored and optimized with considerabletechnical outlay. Further problematical properties of the RTP processare mechanical stresses which it induces and which emanate from atemperature gradient caused by the lamp heating.

In the present example, the oxidation takes place at a temperature of1100° C. over a time period of 60 seconds. This is followed by coolingto 750° C. and renewed heating to 1040° C., at which temperature an NH₃anneal is carried out for 20 seconds. After further cooling to 750° C.,reoxidation takes place at 1170° C. for a period of 60 seconds. In theprocess shown in FIG. 4, the total process time is approx. 400 seconds.However, as has been stated, each wafer is processed individually, whichentails a considerable time outlay for the production of an entire batchof approximately 100 to 150 wafers.

In further known processes, instead of NH₃ NO or N₂O is used in an RTPprocess. Nitriding in an atmospheric oxidation furnace using NO or N₂Oand/or nitriding in an LPCVD furnace using NH₃ as nitriding gas are alsopossible.

U.S. Pat. No. 6,204,125 B1 has disclosed a process for forming anitrided tunnel oxide, in which an oxide layer is formed on a tunnelregion of the substrate, then a nitride layer is formed on the oxidelayer in an LPCVD process, and then a reoxidation process is carried outand finally the nitride layer is removed.

U.S. Pat. No. 5,258,333 has disclosed a process for producing a nitridedtunnel oxide, in which a silicon surface lying above a channel region isfirstly nitrided, then an oxide layer is formed on the nitrided siliconsurface, and next the oxide layer and the nitrided silicon surface areoxidized in order to form a combined dielectric layer. In this process,in particular the nitriding takes place by means of a thermal process inpure ammonia (NH₃).

SUMMARY

Therefore, it is an object of the present invention to provide animproved process for producing a nitrided oxide layer on a siliconsemiconductor substrate which ensures a uniform oxide thickness andinclusion of nitrogen and requires a low thermal budget.

The idea on which the present invention is based consists in the use ofNH₃ as nitriding gas in an atmospheric batch furnace which issimultaneously used for oxidation and for reoxidation.

Compared to the known approach, the production process according to theinvention has the advantage, inter alia, that it is possible to improvethe uniformity of the inclusion of nitrogen compared to conventionalprocesses. Consequently, the subsequent reoxidation likewise leads to animproved uniformity of the overall oxide thickness, since the oxidegrowth is decisively dependent on the degree of nitriding.

According to a preferred development, the nitriding step takes place inan NH₃ atmosphere at a temperature of approximately 850-950° C.

According to a further preferred development, the oxidation step takesplace at a temperature of approximately 800-900° C.

According to a further preferred development, the reoxidation step takesplace at a temperature of approximately 900-1000° C.

According to a further preferred development, the first predeterminedtemperature is lower than the second predetermined temperature, and thesecond predetermined temperature is lower than the third predeterminedtemperature, and no cooling is carried out between the temperatures.

Exemplary embodiments of the invention are illustrated in the drawingsand explained in more detail in the description which follows. In thedrawings:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a temperature curve of an exemplary embodiment of theprocess according to the invention for the production of nitrided tunneloxide;

FIG. 2 shows layer thickness range distributions for nitrided tunneloxide which has been produced by means of an RTP process (A) and fornitrided tunnel oxide which has been produced using NH₃ in a batchfurnace in accordance with the above embodiment (B);

FIG. 3 diagrammatically depicts a semiconductor structure with anitrided tunnel oxide in order to illustrate the problem on which thepresent invention is based; and

FIG. 4 shows an example of a temperature curve for a standard RTPprocess used to produce a nitrided tunnel oxide.

Throughout the figures, identical reference symbols denote identical orfunctionally equivalent components.

DETAILED DESCRIPTION

FIG. 1 shows a temperature curve of an exemplary embodiment of theprocess according to the invention for producing a nitrided tunneloxide.

The present embodiment is carried out using a multiplicity of wafers inwhat is known as an atmospheric batch furnace. First of all, thetemperature is increased to a first temperature plateau at a firsttemperature T1 of approximately 850° C., at which an oxidation step iscarried out for approximately 30 minutes. Then, the temperature isincreased further to a second temperature plateau at a secondtemperature T2 of approximately 900° C., at which an NH₃ anneal takesplace for approximately 10 minutes. A further temperature increase leadsto a third temperature plateau at a third temperature T3 ofapproximately 950° C., at which reoxidation is carried out forapproximately 30 minutes.

Unlike in the known processes, all the wafers are in thermal equilibriumduring the main process steps of oxidation, NH₃ nitriding andreoxidation, with the result that the thickness and composite of thelayer produced are more uniform than in the known RTP process, which inturn leads to a considerable improvement in the yield stability. The useof an atmospheric batch furnace of this type typically allows 150-200wafers to be processed simultaneously, and consequently a higherthroughput can be achieved despite an increase in the overall processtime from 400 seconds to approximately 300 minutes.

Typical thickness ranges for the nitrided tunnel oxides are between 6.5nm and 15 nm.

FIG. 2 shows layer thickness range distributions (in %) for nitridedtunnel oxide which has been produced using an RTP process (A) and fornitrided tunnel oxide which has been produced using NH₃ in a batchfurnace in accordance with the above embodiment (B).

It is clearly apparent that in the conventional process the wafers,which are denoted by 1 to 4, all have a layer thickness distributionwhich is approximately four times as great as a corresponding layerthickness distribution of the wafers 1′ to 4′ in the embodiment of theinventive process which has been explained above.

Although the present invention has been described above with referenceto preferred exemplary embodiments, it is not restricted thereto, butrather can be modified in numerous ways.

In particular, the present embodiment can be applied not only tonitrided tunnel oxides but also to other nitrided oxides, for example tonitrided logic gate oxides which lie, for example, in the thicknessrange of 0.8 nm to 5 nm.

What is claimed is:
 1. A process for producing a nitrided oxide layer ona silicon semiconductor substrate, the method comprising the steps of:a) introducing a plurality of wafers into an atmospheric batch furnace;b) carrying out an oxidation step at a first predetermined temperature;c) carrying out a nitriding step at a second predetermined temperature;d) carrying out a reoxidation step at a third predetermined temperature;and e) cooling and removing the wafers from the atmospheric batchfurnace; wherein said oxidation step b) takes place at a temperature of830-870° C.: said nitriding step c) takes place in one of an NH₃ and anNH₃/N₂ atmosphere at a temperature of 880-920° C.: said reoxidation stepd) takes place at a temperature of 930-970° C.: and between steps b),c), and d) no cooling is carried out.
 2. The process according to claim1, further comprising monotonically increasing the temperature betweenthe first and second predetermined temperatures, and monotonicallyincreasing the temperature between the second and third predeterminedtemperatures.